1. Field of the Invention
The present invention relates to a novel catalytic method for preparation of fused ring heterocyclic compounds. More particularly this invention relates to a process for catalytic dehydrogenation of certain aromatic ether compounds having a hydrogen atom in at least one of the positions ortho to the ether linkage of each of the aromatic groups, to prepare certain fused ring heterocyclic compounds wherein the heterocyclic ring is a five-membered ring oxide.
2. Prior Art
Fused ring heterocyclic compounds such as dibenzofuran, substituted dibenzofurans, and similar compounds including five-membered ring oxides are useful materials. For example, dibenzofuran, also known as diphenylene oxide, and derivatives thereof are useful intermediates for preparation of dyestuffs, stabilizers for organic materials, and other useful chemicals. In addition, dibenzofuran is useful as a functional fluid, as a high boiling solvent for various organic materials, and as has been shown by Pope in Chem. Weekbl., 61, 598-600 (1965), the eutectic mixture of dibenzofuran and biphenyl may be used as a heat transfer agent.
Commercial use of these materials has been hampered however, by reason of high costs, presence of impurities, and general unavailability. Commercially available dibenzofuran for example is presently obtained from coal tar and frequently contains impurities such as acenaphthene and fluroene which cannot conveniently be separated by commercially suitable methods such as washing, recrystallization, or distillation. In some cases dibenzofuran having greater purity as compared to that obtained from coal tar may be synthesized, but the prior art methods are costly because of expensive starting materials, complicated procedures, low yields or the like. For example, dibenzofuran has been obtained in a 30% yield by Graebe and Ullman [Ber. 29, 1877 (1896)] by diazotization and subsequent hydrolysis of 2-aminodiphenyl oxide, while Tauber and Halberstadt [Ber. 25, 2745 (1892)] obtained a yield of 75% by similar treatment of 2,2'-diaminodiphenyl. These two methods however are not adapted to commercial operation due to the high cost and difficulty of obtaining the starting materials required. Hale and Stoesser (U.S. Pat. No. 1,808,349) obtained dibenzofuran by intramolecular dehydrogenation of orthophenylphenol over a refractory oxide catalyst, but again the high cost of the starting material limits the commercial utility of this route. Cullinane [J. Chem. Soc., 2267 (1930)] obtained dibenzofuran in 20% yield in eight hours by pyrolysis of phenol over litharge at 150.degree. C. The low yield limits commercial utility of this route. Walsh and Preston (U.S. Pat. No. 3,108,121) reacted diphenyl ether, also known as diphenyl oxide, over a relatively expensive and sensitive 5% platinum on charcoal catalyst to obtain dibenzofuran. Shiotani and Itatani [Angew. Chem. 86 478 (1974)] used a palladium acetate catalyst to produce dibenzofuran from diphenyl oxide but obtained much dimerized product as well.
In summary, the art is replete with teachings showing the synthesis of dibenzofuran, none of which however is believed to be attractive for commercial production of dibenzofuran and substituted dibenzofurans.